Textures disappear when the rendering engine is switched to cycles
Materials and textures work differently in cycles.

In BI, textures for the selected material are defined separately and most often in the properties editor.
In Cycles, textures are defined as part of the material itself and usually in the node editor (see Learning cycles material nodes).

Assuming a typical basic setup (one UV map, one texture, one material), then in cycles it should look something like this:




Make sure you are in the node editor
Make sure you are editing Material nodes
Add an image texture node (&#8679; ShiftA> Textures > Image) and select your image.
The materials section of the properties editor will reflect what you do in the node editor. Editing the material in either the node editor or the properties editor will update the other.




If you have lots of materials you might try an automated conversion script, see How do I convert materials from Blender Internal to Cycles?